In motor vehicles, particularly commercial vehicles, increasingly high demands are made for transferring forces in the drive train. Therefore, using a customary friction clutch for transferring torque from a drive motor to an input shaft of a vehicle transmission leads to structurally ever larger clutch constructions due to the correlation between the radius of the friction surfaces and the torque to be transferred, as well as consideration of the friction capability and the thermal load capacity of the materials used. However, the construction space in modern vehicles is limited by a variety of aggregates and design specifications so that possibilities are sought for limiting the construction size despite the increasing demands.
An increase of the contact forces of the friction surfaces of the friction clutch with the same or smaller radius would lead to higher demands on the actuator system and therefore would require more costly pneumatic or hydraulic control equipment and more complex activation, particularly costly faster electronic regulators for handling the specified control cycles. Higher contact forces with a greatly reduced clutch radius could also lead to undesirable vibration behavior with high-frequency components, or also to juddering vibrations which negatively impact the wear and operating comfort. In addition, new friction materials that are more highly loadable are currently not available, or at any rate, are very expensive.
In order to circumvent this problem, it would appear that a reduction of the construction size of the friction clutch or an increased transfer of torque with the same size clutch without impairing the driving comfort and increasing the risk of premature wear of the clutch is only attainable by drastically reducing the loading phases of the clutch. This can be achieved if the period in which the friction clutch is slipping is kept as short as possible, and the friction clutch is bridged after attaining the frictional connection.
If during travel, that is, not during creeping or idling, the clutch is slip-free while the engine drive shaft and the transmission input shaft are at the same rotational speed, then the friction clutch can be replaced by a rotationally rigid coupling between the engine and the transmission input. The required radius of an engageable clutch for producing such a rigid connection can be small in comparison to the radius of typical friction clutch discs; in principle, the radius can even be reduced to the shaft radius of the shafts to be connected. A reduction in loading of the friction clutch typically used between the engine and the transmission can therefore be attained by using the friction clutch only briefly for synchronizing the two clutch sides, and otherwise producing a form-locking connection between the input side and the output side of the clutch.
A clutch system that fulfills these requirements must therefore comprise a friction clutch and a form-locking clutch, or a coupling clutch, that are configured in parallel in the power flow.
The document WO 2006/110945 A1 discloses such a clutch arrangement in which a form-locking clutch designed as a dog clutch is disposed radially within a friction clutch designed as a multi-disc clutch. A first, outer hydraulic actuating piston that actuates the friction clutch is positioned within a first cylinder. The outer actuating piston for its part actuates a pot-like second cylinder which receives a second, inner hydraulic actuating piston that actuates the dog clutch. Both actuating pistons are fastened to a drive shaft in an axially movable manner. A disc spring device that preloads the actuating piston in a disengaged position is supported between the outer actuating piston and a disc packet, whose outer discs are connected as a clutch input side to the drive shaft and whose inner discs are connected as a clutch output side to an output shaft. The inner actuating piston on the input side supports an arrangement of projecting claws, which correspond to a commensurate output-side arrangement of recesses that is disposed on the output shaft. A first, outer pressure chamber disposed upstream on the face side is provided for pressurizing the outer actuating piston. A second pressure chamber for pressurizing the inner actuating piston is formed between the outer actuating piston and the inner actuating piston, so that the two actuating pistons and therefore the friction clutch and the dog clutch can in principle be actuated independently of each other.
A disadvantage here is that both actuating pistons are components of the rotating system, whereby relatively large undesirable centrifugal forces can act on the bearing and flow of lubricant. In addition, the inner actuating piston must be moved with a displacement of the outer actuating piston in the engaging direction. Conversely, the inner actuating piston can only return to the initial position thereof if the outer actuating piston was previously placed in the initial position thereof. As a result, the activation possibilities are limited.
Furthermore, a clutch arrangement for a distributor gearing of an all-wheel drive vehicle is known from the document DE 42 00 998 A1. A dog clutch and a friction clutch are disposed therein. The dog clutch can be actuated via an engagement member that acts upon a shifting fork, which pushes a first clutch half designed as a sliding collar onto a spline shaft in order to couple to a second clutch half on a drive shaft. This dog clutch serves to connect, as required, a main drive shaft, particularly a rear-axle drive, to a power take-off shaft, particularly a front-axle drive. The friction clutch connected in parallel in the power flow serves only for synchronizing the two shafts. A disadvantage here is that the dog clutch is axially upstream of the friction clutch such that the arrangement has a relatively long construction length. In addition, the design of the actuation of the dog clutch is relatively complex. The functions of a startup clutch for startup, maneuvering, rolling or coasting are not described therein.
The document DE 42 01 234 A1 shows a gear coupling having a shift member with a gearing, that is disposed in a slidable manner on a drive part, and a counter member having a counter gearing, that is disposed in a rigid manner on an output part. Friction surfaces are assigned to the shift member and the counter member, and these surfaces enter into frictional engagement before the toothed connection, in order to synchronize the drive part and the output part. A disadvantage here is that although the friction clutch is functionally upstream of the toothed engagement, the two clutch functions cannot be controlled independently of each other.